Conventionally, there is known a method in which a single crystal sample or the like is prepared using an atmospheric gas that has an oxygen partial pressure controlled by an oxygen partial pressure control unit provided with an electrochemical oxygen pump containing a solid electrolyte (Patent Document 1).
An oxygen partial pressure control unit illustrated in FIG. 9 includes a mass flow controller (MFC) 3 that controls the flow rate of an inert gas coming through a valve 2 to a set value, an electrochemical oxygen pump 4 capable of controlling the oxygen partial pressure of the inert gas coming through the mass flow controller 3 to a target value, and an oxygen sensor 5 for supply gas, which monitors the oxygen partial pressure of the inert gas, which has been controlled by the oxygen pump 4, and supplies the gas to a subsequent process (unit) such as a sample preparation unit.
This unit further includes an oxygen partial pressure setting section 6 that sets a desired oxygen partial pressure value, an oxygen partial pressure control section 7 that compares a monitor value of the oxygen sensor 5 with the set value of the oxygen partial pressure setting section 6 to control the oxygen partial pressure of the inert gas to be sent from the oxygen pump 4 to a predetermined value, and an oxygen partial pressure display section 8 that displays the monitor value of the oxygen sensor 5. It should be noted that the oxygen partial pressure of the inert gas is normally approximately 10−4 atm.
As illustrated in FIG. 10, in the electrochemical oxygen pump 4, electrodes 4b and 4c made of platinum are formed on both the inner surface and the outer surface of a solid electrolytic cylindrical body 4a having oxide ion conductivity. The solid electrolytic cylindrical body 4a is, for example, a zirconia-based solid electrolyte, and is heated by a heater (not shown). The inert gas is supplied in the axial direction from one opening of the solid electrolytic cylindrical body 4a to the other opening thereof. The inert gas is, for example, Ar+O2 (10−4 atm). The DC voltage of a DC power supply E is applied between the electrodes 4b and 4c disposed on both the inner and outer surfaces thereof. When a positive voltage is applied to the electrode 4c disposed on the outer surface and a negative voltage is applied to the electrode 4b disposed on the inner surface to cause a current I to flow, oxygen molecules (O2) within the inert gas flowing through the solid electrolytic cylindrical body 4a are electrically reduced into ions (O2−), and then are released via the solid electrolyte to the outside of the solid electrolytic cylindrical body 4a as oxygen molecules (O2) again. The oxygen molecules released to the outside of the solid electrolytic cylindrical body 4a are discharged along with auxiliary gases such as air. The inert gas Ar+O2 (10−4 atm) supplied to the solid electrolytic cylindrical body 4a is converted, with the oxygen molecules reduced in number, into a processed gas (purified gas) that has the oxygen partial pressure controlled to a target value, and then is fed to the subsequent process (unit).
It should be noted that the oxygen pump 4 of FIG. 10 is capable of performing the pump operation also when a DC voltage having the opposite polarity to the above-mentioned case is applied between the electrodes 4b and 4c disposed on both the inner and outer surfaces of the solid electrolytic cylindrical body 4a. Specifically, when a negative voltage is applied to the electrode 4c disposed on the outer surface and a positive voltage is applied to the electrode 4b disposed on the inner surface, oxygen molecules (O2) in gas, such as air, flowing along the outer surface of the solid electrolytic cylindrical body 4a are electrically reduced into ions (O2−) via the solid electrolyte, and then released via the solid electrolyte to the inside of the solid electrolytic cylindrical body 4a as oxygen molecules (O2) again. In this case, the oxygen partial pressure of the inert gas flowing inside the solid electrolytic cylindrical body 4a is increased, and the inert gas is fed to the outside.
By supplying a gas that has the oxygen partial pressure controlled by such an oxygen pump, it becomes possible to perform crystal growth, alloying, heat treatment, a semiconductor manufacturing process, and the like, under an inert gas atmosphere having a controlled oxygen partial pressure.
Patent Document 1: JP 2002-326887 A